In contrast to B cells, the T cell receptor repertoire is generally considered fixed once fully mature T cells exit the thymus. We recently developed the technology to detect and sequence the message of paired ?? T cell receptors from single cells in mice and humans. This approach permitted two important observations: 1) the proportion of cells transcribing message for two Tcra chains varies dramatically over the course of acute and memory immune responses and 2) clonal lineages of T cells in vivo demonstrate evidence of receptor revision in the periphery. Using innovative in vitro single cell culture platforms, unique in vivo models including conditional RAG-deficient mice, and a valuable panel of longitudinal human samples, we propose to extend these observations to assess the hypothesis that optimal T cell responses to viral infections require an inherently plastic peripheral repertoire. Our three aims test the specific hypotheses that 1) Dual TCR? allele expression regulates TCR signal strength and functional T cell activity by interfering with in frame allele transcription and/or TCR:CD3 assembly, 2) Peripheral TCR revision is induced in most na?ve T cells by strong TCR signaling and 3) Both dual allele expression and revision are required for optimal T cell activity in acute and chronic infections (influenza and mCMV). These studies combine a suite of novel in vitro technology and in vivo models that will result in a new understanding of how optimal antiviral T cell responses are generated and regulated, with broad implications for our understanding of adaptive immunity.